Opiates appear to activate serotonergic neurons in the caudal brainstem that project to the dorsal horn. The terminals of these cells can release serotonin, which has antinociceptive effects, possibly exerted by presynaptically inhibiting release of neurotransmitters from nociceptive primary afferents or by postsynaptically inhibiting the firing of higher- order neurons. Thus it appears possible that opiate analgesia is mediated at least in part by activation of brain stem serotonergic neurons--in particular, those in nucleus raphe magnus (NRM). The present application proposes to examine first the means by which serotonergic neurons are activated by opiates and second, the means by which serotonergic neurons in turn produce antinociception. First we propose to examine the relationship of opioid receptors to serotonergic neurons. We have recently observed that terminals stained for delta opioid receptors surround serotonergic neurons in NRM, and that NRM neurons themselves stain for mu-opioid receptors. In the present application we propose to examine the relationship between opioid receptors and the population of serotonergic cells in NRM that project to the medullary dorsal horn. In addition, we have observed that serotonergic terminals in lamina V are labeled for the delta opioid receptor. In this application we propose to examine whether other opioid receptors are expressed by serotonergic fibers in the dorsal horn. Moreover, we will attempt to determine in electrophysiological experiments whether these receptors alter release of serotonin. Second, we propose to examine the receptors by which serotonin elicits its antinociceptive effects. The latter will be done by anatomically examining the expression of serotonergic receptors among types of neurons that are predominantly or entirely nociceptive. These will include nociceptive primary afferent neurons (corneal and tooth-pulp afferents) and nociceptive dorsal horn neurons (marginal zone neurons in subnucleus caudalis of the spinal trigeminal complex.) In addition, we will again attempt to determine the function of these receptors by using electrophysiological methods. Together these studies will help clarify the mechanism of action of opiate analgesics and may suggest therapeutic approaches for the treatment of pain that by-pass the use of opiates.